4. THE BACKGROUND GALAXIES

Optical images of the extra-Galactic sky show a variety of
galaxies of diverse luminosities, shapes and distances. At
the limit of what can be seen with photographic surveys,
one finds as many as 18,000 faint galaxies per square
degree.
1 With
modern charge-coupled devices one can
look farther out. During the past decade, ultradeep CCD
imaging
2 over
the wavelength range 0.3-1 micron has
revealed a surprisingly dense and ubiquitous population of
faint blue galaxies. Exposing CCD detectors sensitive to
as little as one photon per pixel per minute at the 4-meter
Interamerican Observatory telescope on Cerro Tololo in
the Chilean Andes, we have found more than 300,000 of
these faint blue background galaxies per square degree of
sky. 3
This is the backdrop we exploit to study foreground
concentrations of dark matter. Of course this previously
unknown background of faint blue galaxies also deserves
study for its own sake.

Figure 3 is a CCD color image of a randomly chosen
4.5 x 5.7 arcminutes of sky. These faint blue galaxies are,
in general, too dim for spectroscopic measurement of their
redshifts at existing telescopes. But several indirect
indicators put their redshifts z between 0.5 and 3. We are
probably seeing these galaxies early in their lives, when
star formation was rampant. The resulting abundance of
hot massive stars would shine brightly in the ultraviolet.
Severely redshifted by a journey of billions of years, the
ultraviolet from these young stars would now look like the
blue spectra we see. The unusual blue color and low
surface brightness of these background galaxies is particularly
convenient for the use we make of them: They are
easy to distinguish from the red galaxies of the foreground
cluster whose dark matter we seek to map. We are seeing
the latter at much more advanced ages, when most of their
stars are old and red. Their star-forming gas has long
since been swept away.

Figure 3. Ubiquitous background of
distant faint blue galaxies shows up on this deep CCD survey
image of a random 4.5 x 5.7-arcminute field
of sky. Three different filters were used to
produce the color image. The digital output
reveals more than 300 000 such faint
background galaxies per square degree
of sky. Most of them have redshifts
between 0.5 and 3.

How do we know that most of these faint galaxies have
redshifts of less than 3? Ultraviolet light traversing
hydrogen gas exhibits a sharp spectral cutoff at 912 Å, the
hydrogen ionization threshold. At wavelengths below this
"Lyman break," stellar photons are heavily absorbed by
the interstellar gas of their galaxy of origin. With a
terrestrial telescope one can't see the Lyman break for
most galaxies, because ultraviolet wavelengths shorter
than about 3200 Å don't get through our own atmosphere.
But if the z of a galaxy in question is high enough, the
Lyman break can be redshifted all the way up to accessible
wavelengths. The fact that our observations of the faint
blue galaxies do not show any clear sign of a short-wavelength
cutoff
4 tells
us that few if any of these background galaxies exceed a redshift of 3.

We can't, in general, measure the redshifts of
individual faint blue galaxies. But we do have rough
statistical limits. The faintest galaxies for which spectroscopic
surveys are available show a redshift peak at about
z = 0.4. But the bulk of the faint blue population is over 16
times fainter than these faintest spectroscopically surveyed
galaxies. The strongly lensing foreground clusters
have redshifts up to about 0.4. So we can conclude that
most of the background galaxies lie beyond z = 1. Some
are certainly as far away as z = 2. That is, their light
began its journey to us when the universe was only a third
of its present size.